It is known to code digital image information obtained by dithering an image by combining in a predetermined order the binary dither values of the pixels compared during dithering with the same threshold value of the dither matrix used to form strings. These strings are then compressed.
For example, U.S. Pat. No. 3,967,O52 discloses a method of forming strings using a part of the dithered image. The strings are formed from the binary pixel values corresponding to one and the same threshold value of the dither matrix. These strings are then individually compressed such as by the run length method wherein the numbers of consecutive equal values in the strings are determined.
Using these methods, images having little variation in grey value, i.e., wherein pixels which correspond to the same threshold value of the dither matrix have the same value (1 or 0) over a wide area, leads to an effective reduction in data required for storing or transmitting the picture image information.
Also of interest is U.S. Pat. No. 4,013,828 wherein groups of pixels are dithered to produce patterns of binary pixel values. Each pattern is compared to a pattern dictionary and, if found, is replaced by an associated code word. Patterns not found in the dictionary are added to it and given a code. Patterns that closely resemble other patterns could be replaced by the code word for the resembled pattern. The coding of this information is addressed for use in transmission by transmitting the code and the dictionary.
Transmission coding is also described in Proceedings of S.I.D., vol 17/2, 1976, p.92-101, entitled Data Reduction of Dither Coded Images by Bit Interleaving, by Judice. In this method, each group of pixels that have been processed together during dithering is separately encoded. The binary pixel values are combined into a string in an order corresponding to the numerical order of the dither thresholds they been dithered with. This string is run-length coded.
In U.S. Pat. No. 4,266,249 a method is described for coding image data for transmission in which multiple level pixel values are coded by two circuits. A high and low frequency circuit is provided. In the low frequency circuit a group of pixels is dithered to produce a pattern of binary pixel values. This pattern is coded by comparing it to a very small dictionary of standard patterns and replacing it by an associated code word. Thereafter the high and low frequency data is combined. However, the procedure for combining the data is highly complicated.